Nick, to my knowledge you cannot just vary the amount of starch in the
roots. However, there are varieties of a plant known as Arabidopsis that
naturally don't have a whole lot of starch in their root cells. I don't
think those seeds are commercially available. You may want to contact the
AIMS (Arabidopsis Information Management System)/ABRC (Arabidopsis
Biological Resource Center). The web address is http://aims.cps.msu.edu/
aims/. I hope this helps!